1. Field of the Invention
The present invention relates to a “V” internal combustion engine provided with balancing countershaft.
2. Description of the Related Art
The balancing shaft is a component of an internal combustion engine which is useful for reducing the vibrations caused by engine unbalance which generates reciprocating second order forces and inertia forces. By limiting vibrations, the balancing countershaft can extend the life of the internal combustion engine, and improves both the performance of the internal combustion engine and the driver's comfort.
The main vibrations in an internal combustion engine are not due to the combustion of the mixture in the cylinders, even if the crankcase is particularly rigid, nor are they caused by the rotation of the crankshaft, which is balanced by means of particular counterweights or particular shapes (unless it is designed together with the countershaft to balance the forces which generate the vibrations), but are instead determined by the reciprocating movement of the pistons which accelerate under the bias of the gas, slowing down abruptly and resuming speed by reversing their direction of motion.
Each change of these changes of speed corresponds to a thrust on the crankshaft which tends to shake the crankcase: for example, during the step of combustion, the piston is “shot” as a projectile and is called back by the connecting rod-crank system, which supports the consequent “whiplash” and transmits it, in form of thrusts (named inertia forces) to the engine assembly, this being why the engine is coupled to the vehicle body by means of elastic mounts which damp, but do not cancel out, the vibration disturbance. The piston causes the most violent shaking when the direction of motion is reversed: in each instant of its movement, however, the system balance is disturbed because it is never stationary or at constant speed: indeed, there is always a step of acceleration or deceleration which generates a trust on the crankcase, in turn.
By starting, for example, from top dead center (TDC), i.e. from the point in which the piston stops its upwards stroke and assuming that the engine turns at constant speed, the speed of the piston is high at approximately mid stroke (when it is half way between the two dead centers, slightly displaced towards the top dead center, this depends on the length of the connecting rod with respect to the crank), followed by a slow phase as it approaches the dead center, but the piston acceleration values are maximum at the dead centers and zero at mid stroke (where the speed is maximum); the greater the inertia force (which also depends on the reciprocating moving mass, i.e. the piston first and foremost) and the vibration, the more accentuated the accelerations. This disparity of speed and accelerations, and the consequent inequality of the inertia forces make it necessary to equip even naturally well balanced engines with special balancing shafts (named “countershafts”).
However, the addition of balancing countershafts in an internal combustion engine inevitably causes an increase of cost, weight and dimensions (not only due to the presence of the balancing countershafts but also for the presence of the mechanical transmission which draws the movement of the crankshaft must activate the balancing countershafts).
U.S. Pat. No. 5,564,380 describes an internal combustion engine having: a crankshaft; camshafts that activate intake and exhaust valves; an auxiliary shaft, which rotates in an opposite direction with respect to the crankshaft and is unbalanced so as to act as balancing countershaft; and a mechanical transmission, which receives the movement from the crankshaft and controls the timing by causing the rotation of the camshafts and, at the same time, causes the rotation of the auxiliary shaft. However, in the solution suggested in U.S. Pat. No. 5,564,380 the tappets of one head are more stressed (and thus subject to higher mechanical wear) than the tappets of the other head.